Our studies this past year concerned the lymphotropic human herpesviruses 6 and 7 (HHV-6 and HHV-7). The salient features of these studies can be summarized as follows: 1. We have isolated a new human herpesvirus, HHV-7, from CD4+ T cells purified from peripheral blood lymphocytes (PBL) of a healthy individual. The original RK strain and three additional HHV-7 strains isolated since then were induced from latency in PBL by conditions promoting T cell activation. The virus is distinct from the 6 previously characterized human herpesviruses as revealed by restriction enzyme and blot hybridization analyses. 2. The HHV-7 genome is approximately 145 kb in size. Approximately 43 kb have thus far been cloned. 3. We have developed an indirect immunofluorescence test and Western blot analyses specific for HHV-6 and HHV-7. In a limited survey we have shown that HHV-7 is ubiquitous in humans, in similarity with other human herpesviruses. Seroconversion occurs in childhood albeit at a later age than HHV-6. Studies designed to determine potential association of HHV-7 with human disease(s) are underway. 4. EM examination of HHV-6 and HHV-7 infected cells revealed details in virus egress from the infected cells. This study led to the recognition of specialized intranuclear structures in which the virus appears to acquire its tegument. 5. We have continued our earlier studies showing that T cell activation was required for HHV-6 replication. This work has documented that complete progression in the cell cycle was required for efficient virus replication. We have also shown that IL-2 at high concentrations inhibited HHV-6 replication. The effect may reflect dependence of virus replication on cell cycle and does not appear to be mediated by gamma-interferon. 6. We have found that infection of quiescent cells with cell free preparations of HHV-6 resulted in inhibition of T cell activation by mitogen(phytohemagglutinin) or by antigen (tetanus toxoid). Similar immunosuppressive effect was observed following infection of the cells by herpes simplex virus.